PROJECT SUMMARY Air pollution exposure is an important risk factor for cognitive decline and Alzheimer?s Disease (AD), a common form of dementia affecting 1 in 10 U.S. adults over age 65. Older adults exposed to high levels of air pollution are at greater risk of poor cognitive function and decline as well as dementia and dementia-related pathology. Importantly, outdoor air pollution is ubiquitous and many older adults are exposed to air pollution levels above the recommended federal standards for healthy air quality. Because air pollution is widespread and plays a key role in neuro-degeneration, determining its association with dementia, as well as precursors to dementia, has become a public health priority. Psychosocial stressors in the neighborhood environment may lower the brain?s threshold for neurotoxicity and increase susceptibility to neurodegenerative effects of pollutant exposure at older ages. With additional exposure to neighborhood stressors, air pollution could negatively impact cognitive health even at relatively lower doses than have previously been reported. The proposed study will determine if PM2.5 exposure interacts with disorder and decay, two major neighborhood-based stressors, to impact cognitive decline and dementia risk among older U.S. adults. This project will use prospective data on adults ages 60 and older from the nationally representative Health and Retirement Study (HRS), linked with annual average concentrations of PM2.5, to examine cognitive decline and onset of cognitive impairment consistent with dementia over ten years. The primary objective of this two year project is to test the central hypothesis that exposure to neighborhood stressors exacerbates the adverse effects of air pollution on risk of cognitive decline and onset of dementia.